Friends don't let friends stay in something as stupid as the EU - Credit, public domain

From The Conversation:

Logic puzzle: can you solve this baffling Brexit conundrum?

At a party, the host arranges the guests in a circle and sticks a post-it note onto each of their foreheads. Each guest can see what’s written on everyone else’s post-it, but not their own.

“You are all Brexit negotiators,” says the host. “Some of you are representing the UK and some the EU, but all of you are negotiating for one or the other and there will be at least one negotiator for each side. The side you are representing is now written on your forehead. UK or EU – does anyone know which they are representing?”

Everyone shakes their head and says they don’t know.

“Does anyone know now?” asks the host.

Again, everyone shakes their head and says “I don’t know”.

But the host persists: “Now does anyone know?”

This cycle continues, to the perplexity of all and the annoyance of some. On the host’s sixth time of asking, the guests, by now wishing they’d gone elsewhere for the evening, again answer, for the sixth time, that they don’t know. But as soon as they’ve done so, some of the guests say, truthfully, “Now I know! I’m EU!”

Given the above facts, how many EU negotiators are there at the party? And what is the minimum number of guests? (Assume that all the guests have reasoned as well as they could have.)

The EU side would be foaming at the mouth with federasty. Obviously.

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I’m trying to remember, I think it’s a power-two thing (two options: UK, EU), so I think there’s 32 people there (2 ^ (6-1)).


The answer is seven, the same as the number of rounds at which someone knows the answer. On round one someone will say “I’m EU” if he/ahe cannot see an “EU” on any other forehead; on round two if someone can only see one “EU” he/she willknow that he/she must be the second or the other person would have spotted their uniqueness and so on …


First round everyone knows that there is at least one of each. Had there been only one of either he would have the answer. No-one did hence they all now knew the minimum was two of each. With each successive round the minimum raised by one of each. On what was effectively round seven some people observed that there were only six EU post-its visible and hence that they must be one of the seven EU negotiators. The Brit negotiators could make no such deduction, hence they must have been more numerous. My answer therefore is seven EU negotiators and… Read more »